Commander Keen: Extinction is KeenRush's fifth mod, made for Keen 1. The core of the gameplay is lemm'sauto-scrolling screen patch, which forces Keen to move right all the time. Other patches are by KeenRush. The mod has all-new enemies with behaviours that may surprise the player. Graphics are sparse and the mod uses only a handful of tiles. Keen has infinite lives but no raygun.

In every level Keen must collect 5 snowflakes (you can see one in the in-game picture on right) to be able to finish the level and move on to the next. If you do not have the snowflakes when you touch the exit you will simply die and have to play the level again.